Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy. In direct thermal thermography a visible image pattern is formed by image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density. Most of the "direct" thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
U.S. Pat. No. 3,846,136 discloses that the thermosensitive or other hardenable layers in thermographic materials can be hardened with organic or inorganic hardening agents such as aldehydes and blocked aldehydes, ketones, carboxylic and carbonic acid derivatives, sulfonate esters, sulfonyl halides and vinyl sulfonyl ethers, active halogen compounds, epoxy compounds, aziridines, active olefins, isocyanates, carbodiimides, mixed function hardeners and polymeric hardeners such as oxidized polysaccharides like dialdehyde starch and oxy-guargum and the like. However, this disclosure is merely an invitation to experiment, because almost all hardening agents known for gelatino silver halide emulsion photographic-sensitive materials processed by wet treatment are enumerated and there is no example using them.
WO 95/12495 (=U.S. Pat. No. 5,710,095) discloses a method of recording an image by image-wise heating a recording material, the recording material comprising on the same side of a support, called the heat sensitive side, (1) one or more layers comprising an imaging composition essentially consisting of (i) a substantially light-insensitive organic silver salt being in thermal working relationship with (ii) a reducing agent, and (2) at the same side covering the imaging composition a protective layer, characterized in that the image-wise heating proceeds with a thermal head contacting the heat-sensitive side and through the protective layer mainly comprising a cured polymer or cured polymer composition. In a preferred embodiment thereof, the protective layer contains hydrophilic polymers having active hydrogen atoms at least part of which has reacted with hardening agents selected from the group consisting of polyisocyanates, polyepoxides, aldehydes and hydrolysed tetraalkyl orthosilicates.
U.S. Pat. No. 5,468,603 discloses a thermographic emulsion layer containing an organic silver salt, a polyisocyanate, a binder, e.g. BUTVAR.TM. B-79, and a large selection of reducing agents, with a sterically hindered bisphenols being preferred, provided with a protective layer comprising a polymeric fluorinated surfactant. DESMODUR.TM. 3300 is disclosed in U.S. Pat. No. 5,468,603 for photothermographic emulsion layers.
U.S. Pat. No. 5,468,228 discloses a thermographic emulsion layer containing an organic silver salt, a binder, e.g. BUTVAR.TM. B-79, a substituted propene compound and a large selection of reducing agents, with a sterically hindered bisphenosl being preferred. DESMODUR.TM. 3300 is disclosed in U.S. Pat. No. 5,468,228 for photothermographic emulsion layers.
In coating thermographic materials from solvents with protective layers it is desirable for economic and logistical reasons to coat the organic silver salt-containing layer simultaneously with a protective layer. This requires hardening of both layers to prevent significant interfacial mixing thereof. However, the use of such hardeners was found to affect significantly the image gradation of the resulting materials.